Transforming growth factor beta (TGFβ) has been demonstrated to play an important role in the regulation of the immune response, primarily through its suppressive function towards cells of the immune system. TGFβ is a suppressor of antigen-specific T cell proliferation at least through reduction of the cell-cycle rate, as opposed to induction of apoptosis. In particular, TGFβ acts on cytotoxic T lymphocytes (CTLs) to specifically inhibit the expression of at least five cytolytic gene products: perforin, granzyme A, granzyme B, Fas ligand, and interferon gamma that are important for CTL-mediated tumor cytotoxicity (Thomas and Massagué, 2005).